The invention relates generally to micro-sized devices that can be used to manipulate very small quantities of gases or liquids, or as conductive probes or neural stimulators, or to provide a plurality of test surfaces for gene probes, and more specifically to methods for fabricating arrays of such micro-sized structures.
In scientific fields such as micro-fluidics and combinatorial chemistry it is often necessary to manipulate very small quantities of gases or liquids is required, for example volumes as small as a picoliter or so. Understandably it is difficult to fabricate hollow micro-needles dimensioned for such tasks, especially where needle lengths exceeding 1 mm are required. Without limitation, such micro-needles can or should find application in the dispensing and manipulation of DNA samples, in fabricating so-called gene IC chips for example.
Conventional DNA sampling is often carried out using an array of perhaps forty-eight needles, a relatively low number. The needles in the array are typically rigid, made of stainless steel, and are rather expensive. Various of the needles are exposed to a solution that may contain DNA or other biological or chemical materials, and the needle tips are then urged against regions on a substrate. Substrate regions may be coated with various DNA, biological, or other chemical samples, with which the materials transported via the needles may react. A very real problem is avoiding fracturing the needles as they meet the typically rigid glass substrate, and avoiding cross-contamination between various sample solutions. Often the needles are individually spring loaded, and the relatively non-dense arrays help guard against cross-contamination. But it would be desirable to use probe arrays containing thousands or tens of thousands or more of individual needle-probes. It would be highly useful to be able to provide a substrate useable with such dense arrays of needle probes, or indeed useful with existing rigid low density probes, which substrate would be somewhat flexible and would define different test regions.
In a standard gene chip or gene cell procedure, the chips are patterned with an array of spots of distinct DNA samples. The chip is then exposed, for example by soaking in a solution containing cDNA samples taken from a piece of tissue, a tumor perhaps. More specifically, RNA is taken from the tissue and converted into corresponding DNA, which is then replicated via PCR. Thus in standard gene chips, when the needles deposit their samples the chip will be bare or empty. But in combinatorial chemistry, or in material science applications, samples may already be present.
In other fields, small preferably solid micro-needles are desired as probes to sense electrical signals, or as probes to apply stimulation electrical signals, such as from neural tissue or other complex media. Without limitation, suitably sized hollow and/or solid micro-needles could find application in neural stimulation, sensing, sampling, injection, light absorbing sensors, and light emitting surfaces. Other applications should include micro-scale or nano-scale xe2x80x9cstampsxe2x80x9d such as so-called cookie-cutter tools used in micro-scale or nano-scale applications. Appropriate micro-needles should also find application as electron emitters, for example in high electric field avalanche multiplier structures. Appropriate micro-needles should also find use in gaseous-based or liquid-based detectors of ionizing radiation. Appropriate hollow micro-needles may also be used as extrusion nozzles in micro-sale or nano-scale applications.
Thus there is a need for a method to produce micro-needles, and arrays of such micro-needles, that may be hollow or solid in cross-section, and that preferably are electrically conductive, yet can be fabricated to be electrically isolated from each other. The method should produce micro-needles with diameters that can range from a few nanometers to several millimeters, with height/diameter aspect ratios ranging from under unity to more than one thousand. In cross-section, the resultant mold or micro-needle structures may have any configuration, including circular, rectangular, triangular, line segment, filled-in polygons, hollow polygons, etc. Depending upon materials used, e.g., thermal oxide, tungsten, etc., the resultant structures should function at temperatures exceeding 1,000xc2x0 C.
Further, there is a need for a method to produce preferably flexible substrates that can define a dense array of individual surfaces suitable for micro-needle probing in a gene chip application. The flexibility of such substrates would minimize needle probe breaking and the individual surfaces formed on the substrate would minimize cross-contamination of samples.
Preferably a method of producing such structures should use techniques and equipment presently available for fabrication in the semiconductor industry. Once fabricated, such micro-needles, arrays, and substrates should find use in any or all of the various applications noted above.
The present invention provides such fabrication method for producing such micro-sized structures and arrays of such structures.
Solid and hollow micro-needle structures and arrays of such structures are fabricated by forming an electrical insulating layer on a standard silicon wafer. At least one wafer surface is polished, and is array-patterned. Material is then etched away from the bulk of the wafer through the pattern to form cavities where micro-needles are desired to be formed. The etch depth extends into the wafer a length corresponding generally to a desired length of the micro-needles to be formed. The etching forms a plurality of cavities extending from the wafer top into the bulk of the wafer. The cavities may taper inward or outward, be cylindrical or indeed have an hourglass or other shape, depending upon the techniques used to remove material from the wafer bulk.
The cavity cross-section dimensions may range from a few nanometers to several millimeters, and the height/diameter aspect ratios may range from less than one to greater than one thousand. Pitch density may range from about 1 xcexcm to about 1 cm. In cross-section, the mold cavities may have any desired configuration including circular, rectangular, triangular, line segment, filled-in polygons, hollow polygons, etc. Different mold cavities within an array may have different dimensions and shapes, if desired. Cavity depth may range from about 1 xcexcm to the thickness of the wafer, perhaps 1 cm or so.
A sacrificial layer of polysilicon may now be deposited into the newly etched holes. The profile of what will be micro-needles is substantially vertical except near the distal tip, and such use of polysilicon can substantially increase the final height/diameter aspect ratio of the finished micro-needles.
At this juncture, the substrate may be used as a mold, in that the substrate defines an array of cavities extending into the substrate bulk. If electrically conductive micro-needles are to be formed, the cavities may be filled with an electrically conductive material, e.g., gold, tungsten, copper, nickel, perhaps aluminum, doped polysilicon. If it is desired to produce non-conductive micro-needles, the cavities may be filled with a non-conductive material, e.g., glass. In either case, substrate bulk material is then removed from the bottom of the wafer upwards to expose a desired length of the now conductive material filled micro-needles. If desired, electrically conductive contact pad, traces and wire bond pads may be fabricated to couple electrical signals to individual ones of the micro-needles in the resultant array. The micro-needles adopt the size and shape of the cavities formed in the mold.
If desired, the mold formed in the wafer bulk may be used to form a flexible substrate for DNA gene cell applications, in which individual elevated substrate regions or plateaus are defined by the cavities in the substrate. Cavity depth would determine plateau height, which is to say well depth if the structure is inverted, and patterning would determine array density and the cross-sectional configuration of various plateaus. The cavities formed in the substrate would be filled with an appropriate flexible material, for example, polydimethyl siloxane (PDMS). The substrate bulk would then be removed such that what is left is a honeycomb-like array of wells that preferably include concave regions separated from other wells by elevated perimeter walls projecting upward from the PDMS surface. Such PDMS material is flexible and would not break or deform DNA probes, and when used with micro-probes according to the present invention could provide array densities of tens of thousands of micro-needle probes that contact individual plateau regions on a flexible membrane substrate, also formed according to the present invention. Further, the present invention provides a method to produce cantilevered high density probes, suitable for contacting test pads and the like on integrated circuits.
Hollow micro-needles may be fabricated as well as solid micro-needles. However at the starting phase, the lower surface of the wafer is coated with silicon nitride (SiN) or the like. Etching through the patterned upper surface is continued through the wafer thickness, down to the upper regions of the SiN layer at the wafer bottom. Dimensions and configurations of the cavities may be as described above. Thermal oxide is then grown to cover all of the wafer including the sidewalls of the cavities formed, except of course for the SiN covered bottom region of the cavities. The SiN is then etched or otherwise removed, and substrate bulk material is removed from the bottom-up to expose a desired length of hollow glass micro-needles. If desired, the micro-needles could be coated to be electrically conductive, to present an array of hollow conductive micro-needles. Such micro-needles could be combined with micro-fluid pumps, fluidic channels, reservoirs, etc., for use in various applications.
Transistors and integrated circuits may be formed on the same wafer as the array of micro-needles and/or micro-probes. In applications requiring micro-needle dispensed fluids, tiny fluid reservoirs and/or fluid carrying channels may also be formed on the wafer. Application of electrical potential between the wafer substrate and micro-needles can encourage the fluid to prefer the substrate to a micro-needle, thus promoting fluid flow.
Additional structure may be added to the micro-needle wafer structure to build more complex systems. For example, fabricating electrode sets on parallel surfaces and coupling electrical drive signals to the electrodes can result in the controlled vertical displacement of selected micro-needles. In other applications, fluid may be dispensed onto a substrate or slide using a micro-needle fabricated according to the present invention, and application of electrical potential can assist in promoting the desired flow.
Non-biologic and genetic research applications can include fabricating capillary micro-needles for use in colloid micro-thruster arrays, for space propulsion application. Such colloidal micro-thrusters may comprise a charged colloid drop or jet emitter, fabricated from a micro-needle as described herein, following by a stack of one or more extractor plates. The micro-needle is centered above an opening in the first extractor plate, which serves primarily as a high voltage structure that induces injection of colloidal drops or jets from the emitter. The second and subsequent plates may be used to increase velocity (and thus thrust) provided by the emitted colloid drops, or may be used to modify trajectory and/or direction of the emitted drops or jets. The ability to thus change drop or jet trajectory and/or direction provides a vectoring capability for a propulsion system. Applications for such systems are disclosed in Cappelli, M. and Pranajaya, F. xe2x80x9cColloid Propulsion Development For Use in Microsatellite Applicationsxe2x80x9d, research report, Stanford Univ., May 1999.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings.